Biomedical Engineering Reference
In-Depth Information
Thus, one of the most important in vitro validation techniques for UHMWPE
materials is wear testing. Pin-on-disk (POD) wear testing is a common com-
parative method to quantify the wear rate of UHMWPE load-bearing materials
(Wang et al., 1998b; Muratoglu et al., 1999; Saikko et al., 2001; ASTM, 2006b).
Cylindrical pins machined of UHMWPE are articulated against a smooth
counterface disc made out of implant-finish metal, commonly cobalt±chromium
alloy (CoCr, Ra ~ 0.05m) in a bidirectional path (Bragdon et al., 2001). This
motion is especially designed after the hip joint, where the principal arc of
motion in the flexion-extension direction is accompanied by transverse motions
in the abduction/adduction and internal/external rotation axes at the articulating
surface (Ramamurti et al., 1998). Wear is determined gravimetrically as
UHMWPE does not absorb significant amounts of testing medium during the
1±2 million cycles over which POD tests are typically performed. The wear rate
is calculated as the linear regression of wear against the number of cycles
disregarding the first 200 000±500 000 cycles where creep deformation of the
pin occurs and surface asperities are eliminated.
To obtain more clinically relevant results, simulators have been designed for
wear testing (Scales et al., 1969; Dumbleton et al., 1972; Shaw and Murray,
1973; Walker et al., 1997). Simulators have been designed to match the in vivo
wear rate, wear debris size and shape of UHMWPE as well as the surface
morphology observed in explanted components by replicating in vivo attributes
such as component positioning, temperature, loading and kinematics of normal
gait (Paul, 1966; Johnston and Schmidt, 1969; Bergmann et al., 1993; Ramamurti
et al., 1996, 1998), all of which are known to affect the wear rate of UHMWPE.
The simulator tests are typically carried out at 1±2 Hz, at 37±40 ëC bovine serum
to simulate joint temperature. Wear is assessed gravimetrically and can be
measured also by a volumetric method using a coordinate measuring machine,
which can measure the three-dimensional shape of the articular surface. When
measuring wear volumetrically, creep deformation of UHMWPE has to be
separated from wear; this can be done by melting the component after testing is
completed as UHMWPE has excellent shape memory and creep deformation can
be eliminated by re-melting the component (Muratoglu et al., 2005b).
Limitations of simulators are the choice of kinematics, which is usually limited
to normal gait kinematics in contrast to a complex variety of activities of daily
living. Another factor affecting wear in the simulator is the testing environment.
While bovine serum is the accepted analogue for synovial fluid, dilution of
bovine serum with water to obtain protein concentrations relevant to in vivo
conditions is commonly used. However, wear of conventional (gamma sterilized)
UHMWPE has been shown to increase with bovine serum dilution (Wang et al.,
1998a). At high water concentration, non-physiological wear mechanisms can
dominate the wear process (Liao et al., 1999). Other factors that may affect the
outcome are surface finish of the metallic counterface, stress levels and loading
mode (dynamic vs. constant) (Haider, 2009).
